Spectroscopy is used for analytics, environmental control, quality control, and process control. Radiant energy is directed towards a substance and the absorption, diffuse reflection, dispersion, fluorescence, or refraction of the substance can be measured in order to determine the substance and its properties.
The prior art includes two major types of spectroscopy: (1) Absorption spectroscopy; and (2) Reflectance spectroscopy.
Absorption Spectroscopy
Conventional absorption methods are used for the detection of absorbing substances in measurement volumes, such as liquids, gases, or solids.
Radiation of a defined wavelength is coupled into the measurement volume. On its way through the measurement volume, the coupling radiation is attenuated by absorbing substances within the measurement volume. After a defined distance, the coupled radiation is again decoupled and directed to an optoelectronic receiver, which records the attenuated intensity I. The quotient of the attenuated and unattenuated intensity I.sub.o is the transmission T: EQU T=I/I.sub.o =exp (-.alpha..sub.T .chi.) (1)
This law from Bouger-Beer-Lambert describes the relationship between transmission and the total absorption coefficient .alpha..sub.T (for the sake of simplicity dispersion has been ignored here). The term .chi. is the distance which the coupling radiation travels in the measurement volume. (BERGMANN and SCHAEFER: Textbook of Experimental Physics. Optics. Berlin-New York, Walter de Gruyter, 1993 and SCHMIDT, W.: Optical Spectroscopy. Weinheim-New York-Basel-Cambridge-Tokyo, VCH Verlagsgesellschaft, 1994.)
A special absorption method is based on the principle of evanescent wave fields or attenuated total reflection (ATR). Here, radiation is coupled into a light conducting solid, e.g., ATR crystal or an optical waveguide and after passing through a defined distance decoupled again. The optical waveguide is in contact with the measurement volume to be investigated. In the optical waveguide, the coupling radiation is totally reflected at the boundary surface with a measurement volume, whereby a small portion of the radiation penetrates into the measurement volume (evanescent wave) and interacts therewith. Thus, the coupling radiation is attenuated. This attenuation is measured. The classical relationship in Formula (1) applies. (BERGMANN and SCHAEFER: Textbook of Experimental Physics. Optics. Berlin-New York, Walter de Gruyter, 1993)
In the case of measurement volumes with very low optical densities (e.g., gases), the distance traveled by the coupled radiation in the measurement volume is increased in order to obtain evaluable signals. Long paths may be realized, for example, with the help of reflecting elements. (BAUMBACH, G.: Air Cleaning. Berlin-Heidelberg-New York, Springer Verlag, 1992) In DE 4104316A1, an internally reflecting spherical cell is introduced, in which the coupled radiation is reflected back and forth multiple times and then decoupled again and directed to a receiver. In DE 4124545A1, a gas absorption cell is described.
Recently, a method for the determination of total absorption has been proposed. Rather than measuring the coupling radiation attenuated after traveling a defined distance, the interaction radiation (fluorescence and dispersion) generated by the coupling radiation is measured. The special characteristic of this method is that the coupled radiation is almost completely absorbed by the measurement volume after a long path. (DD 301 863 A7. DE 43 37 227 A1, and MITTENZWEY, K.-H., J. RAUCHFUSS, G. SINN, H.-D. KRONFELDT: A new fluorescence technique to measure the total absorption coefficient in fluids. Fres. J. Anal. Chem., 354 (1996) 159-162 as well as MITTENZWEY, K.-H. & G. SINN: MPSS: A new scattering technique for measuring the total absorption in fluids. Appl. Spectr. 51 (1997) 2, 82-85)
Reflectance Spectroscopy
Reflectance spectroscopy combines diffuse reflection and specular reflection, i.e., directed reflection.
(a) Diffuse Reflection
Diffuse reflection R is the diffuse reflection of radiation on a material i.e., measurement volume. It is a measure of the intensity of the photons reflected opposite the direction of incidence. These are scattered photons in the classical sense. Diffuse reflection is determined by the dispersion power (dispersion coefficient .beta.) and absorption power (total absorption coefficient .alpha..sub.T) of the measurement volume. For the sake of simplicity, absorption will dominate in the following description of diffuse reflection. The theory of Kubelka and Munk is used for the mathematical description of diffuse reflection. In an infinitely extended measurement volume (e.g., a deep body of water), the diffuse reflection R.sub.S is proportional to the quotient of the dispersion coefficient and the absorption coefficient as follows. EQU R.sub.S .about..beta./.alpha..sub.T (2)
If the radiation incident in the measurement volume also generates fluorescence, the diffuse reflection is determined not only by the dispersion but also by the fluorescence power, which is characterized by the product of the fluorescence quantum yield Q.sub.F and the absorption coefficient of the fluorophore .alpha..sub.F of the measurement volume (Q.sub.F .alpha..sub.F) The fluorescence contribution R.sub.F to the diffuse reflection of extended measurement volumes is decisively controlled by the quotient shown in formula (3) below. EQU R.sub.F .about.Q.sub.F .alpha..sub.F (.lambda..sub.E)/[.alpha..sub.T (.lambda..sub.E)+.alpha..sub.T (.lambda..sub.F)] (3)
where .lambda..sub.E and .lambda..sub.F are the wavelengths of the incident radiation and fluorescence, respectively. In many cases of transmitting measurement volumes, the absorption at the wavelength of the incident radiation is greater than the absorption at the fluorescence wavelength (e.g., eutrophic surface water), in which case, formula (3) changes to formula(4): EQU R.sub.F .about.Q.sub.F .alpha..sub.F (.lambda..sub.E)/.alpha..sub.T .lambda..sub.E) (4)
Formulas (2) and (4) are characterized by the same mathematical structure. The diffuse reflection is in both cases first proportional to the dispersion or fluorescence power and also inversely proportional to the total absorption.
Diffuse reflection spectroscopy is used, for example, basically for remote sensing and is applicable to both very optically dense and transmitting measurement volumes. Examples of optically dense measurement volumes include diffusion reflection measurements on vegetation (leaves or needles), to determine their physiological condition, or measurements on soils to determine moisture and structure.
Examples of transmitting measurement volumes include atmosphere, bodies of water, and oceans. Comparatively simple relationships are present when the incident radiation (global radiation, lidar) can die out in the measurement volume, e.g. in bodies of water, the incident radiation does not reach the bottom. KORTUM, G.: Reflection spectroscopy. Berlin-Heidelberg-New York, Springer Verlag, 1969 and COLWELL, R. N.: Manual of remote sensing. Falls Church, The Sheridan Press, 1983.
(b) Reflectance
Reflectance spectroscopy is used preferably for the investigation of solid surfaces. In reflectance spectroscopy, the radiation directly reflected or directed from a surface is analyzed (reflectance law), which gives information concerning the spectral reflectance. In the analysis of diffuse reflection R from transmitting solid, liquid, and gaseous measurement volumes, the specular reflectance occurring at the boundary surface with the measurement volume is as a rule a disturbance variable, that is filtered out by suitable measurement arrangements.
The specular or directed reflectance R.sub.G is dependent, among other things on the refractive index n of the measurement volume. Since, in many cases, the measurement volume absorbs, the refractive index decisive for reflectance is also determined by the absorption power of the measurement volume. The refractive index R.sub.G is made up of a real component and an imaginary component (complex number): EQU R.sub.G =((n-1)/(n+1)).sup.2 (5)
with n=n.sub.Real+n.sub.Imaginary. The formula (5) is a simplified representation for the air/measurement volume boundary with perpendicular incident radiation. The refractive index is determined in practice goniometrically or interferometrically. (BERGMANN and SCHAEFER: Textbook of Experimental Physics. Optics. Berlin-New York, Walter de Gruyter, 1993 and SCHMIDT, W.: Optical Spectroscopy. Weinheim-New York-Basel-Cambridge-Tokyo, VCH Verlagsgesellschaft, 1994)
Advantages and Disadvantages of Absorption and Reflectance Spectroscopy
A significant advantage of classical absorption spectrometry compared to spectrometry measuring fluorescence and dispersion is that in classical absorption spectrometry, the coupled radiation falls directly on the receiver instead of indirectly on the receiver; thus, significantly more photons are available for measurement. High signal/noise ratios result from the large number of photons measured in classical absorption spectrometry. In classical absorption spectrometry, radiation sources with low photon fluxes and simple semiconductor receivers can be used because of the high signal/noise ratios. Consequently, the technical equipment outlay is relatively low. A significant disadvantage of classical absorption spectrometry is the relatively low sensitivity as a result of the exponential relationship between the coupling radiation attenuated by the measurement volume and the absorption coefficient, as shown in formula (1).
A significant advantage of diffuse reflection spectroscopy is that the relationship between the total absorption coefficient and the diffuse reflection is inversely proportional, as shown in formulas (2)-(4). Thus, diffuse reflection is more sensitive than the classical absorption spectrometry. Moreover, diffuse reflection includes information concerning the dispersion power and fluorescence power of the measurement volume. However, it is disadvantageous with diffuse reflection spectroscopy that the relationship between R and .alpha..sub.T,.beta.,Q.sub.F .alpha..sub.F is ambiguous. This results in the fact that an exact separation of absorption power, dispersion power, and fluorescence power, respectively, is difficult and in many cases impossible. Moreover, the use of diffuse reflection for a sensitive determination of the absorption power of transmitting measurement volumes is bound to extended measurement volumes, since the radiation dies out only after relatively long paths in the measurement volume (e.g., 10-230 cm with typical absorption coefficients for surface water of from 1-23 m.sup.-1). In samples with smaller layer thicknesses (e.g., classical cells), determining the absorption power is impossible. Furthermore, with the irradiation of light bundles with a finite cross-section in the extended measurement volume (e.g., lidar), the photometric distance law has a disruptive effect on the signal-to-noise ratio.
An advantage of reflectance spectroscopy is that the intensity of the radiation specularly reflected at the boundary surface with the measurement volume provides data concerning the refractive power, which is substance-specific. Using the refractive power, it is even possible to characterize substances which are completely incapable of absorption. However, it is disadvantageous that the specular reflectance also depends on the absorption power of the measurement volume and is thus ambiguous.